Just to out rule a few variables.....
Tyre pressures the same across each axle?
A brake caliper binding?
Has the recent hot spell gouged lorry ruts into your local roads?
Mine always shows dead on on alignment rigs but the only time it ever pulls dead straight, hand off the steering wheel, is on a dead flat road......which are few and far between! There is always a varying amount of left or right tug, depending on surface wear and built-in drainage camber etc etc, but obviously keeping the car straight shouldn't be a battle with the steering wheel......just a slight correction here and there. If you're forcibly having to centre the wheel, then yeah, that's not right
I think I'll call upon me ol' mate Occam and his Razor on this one....... and the razor being the alignment STILL isn't right! I.e. the simplest answer is usually the right one